1. Field of the Invention
The present invention relates to an electromagnetic switch particularly adapted to be mounted on a starter motor.
2. Description of the Prior Art
FIG. 3 is a side elevational view in cross section showing a starter motor equipped with a conventional electromagnetic switch, and FIG. 4 is a side elevational view of the conventional electromagnetic switch illustrated in FIG. 3, showing the upper half thereof in cross section. In these figures, reference numeral 1 designates a front bracket connected with a casing C in which a starter motor M is accommodated.
Mounted on the front bracket 1 is a conventional electromagnetic switch 2 having a movable iron core assembly 4 constituting a movable magnetic pole. The movable magnetic core assembly 4 has a hook 5 operably connected with a shift lever 3 which is pivotally supported by a projected seat 1a on the inner surface of the front bracket 1 and a spacer 7 which is mounted through a grommet 6 on the casing C, the spacer 7 being urged toward the projected seat 1a under the resilient action of the grommet 6. Mounted on the front bracket 1 is a cylindrical housing 8 which is formed at its one end with a radially opened recess or socket 8a into which a stationary iron core 9 constituting a stationary magnetic pole is fitted and fixed thereto as by caulking together with a cap 11 of a switch cap assembly, generally designated by reference numeral 10. A packing 28 is interposed between an end surface of the cap 11 and the adjacent side surface of the stationary iron core 9. Housed in the cylindrical housing 8 is an electromagnetic coil 12 which is composed of an attraction coil (called P coil) and a support coil (called H coil) and which is wound around a bobbin 13 moulded of a resinuous material. A bushing 14 is mounted on the inner surface of the bobbin 13 and has one end thereof fitted on the radially outer periphery of the annular extension 9a axially extending from and integrally formed with the stationary iron core 9, and the other end thereof fitted in an opening 15 formed in the housing 8 at its front end adjacent the front bracket 1.
Slidably fitted in the bushing 14 is a cylindrical-shaped movable iron core 16 having a cylindrical bore 17 into which the hook member 5 is fitted to be axially slidable with a coiled compression spring 18 being disposed under compression between a retainer ring 19 fixedly secured to an end of the movable iron core 16 and a flanged end 5a of the hook member 5 for resiliently urging the hook member 5 against the bottom or inner end of the bore 17. The movable iron core 16 is integrally formed at its rear end (the lefthand end in FIG. 3) with a cylindrical shaft member 20 on which is mounted an annular movable contact 22 through the intermediary of an insulating sleeve 21 of an electrically insulating material, the sleeve 21 being slidably fitted over the shaft member 20 so that the movable contact 22 is electrically insulated from the shaft member 20 by means of the insulating sleeve 21 and is slidable together with the insulating sleeve 21 along the outer periphery of the shaft member 20.
The movable contact 22 is urged by a biasing spring 23 toward an annular stationary contact 26 fixedly mounted on the cap 11 in face-to-face relation to the movable contact 22, the biasing spring 23 being formed of a coiled compression spring and disposed between a stepped shoulder on the shaft member 20 and the adjacent end of insulating sleeve 21. The movable contact 22 is also electrically insulated, in addition to the insulating sleeve 21, from the shaft member 20 by means of an insulating washer 24 which is fitted on the shaft member 20 and fixed thereto by means of a stop ring 29.
Disposed between the stationary iron core 9 and the movable iron core 16 is a return spring 25 in the form of a coiled compression spring for biasing the movable iron core 26 in the direction away from the stationary iron core 9 so that the movable contact 22 mounted on the shaft member 20 connected with the movable iron core 16 is biased by means of the return spring 25 in the direction away from a stationary contact 26 firmly secured to the cap 11. The stop ring 29 mounted on one end of the shaft member 20 serves to prevent the insulating sleeve 21, the movable contact 22 and the insulating washer 24 from falling out from the shaft member 20.
In operation, when the electromagnetic coil 12 including the P coil and the H coil is energized, upon closing of a key switch (not shown), by an external power source (not shown), a magnetic flux is produced which passes through a magnetic circuit constituted by the housing 8, the movable iron core 16, the clearance between the movable and stationary iron cores 16 and 9, and the stationary iron core 9, and the movable iron core 16 is magnetically drawn toward the stationary iron core 9. As a result, the return spring 25 is compressed so that the movable contact 22 is caused thereby to move toward and contact with the stationary iron core 9 to energize the starter motor M. On the other hand, when the movable contact 22 contacts the stationary contact 26, the biasing spring 23 is similarly compressed so as to provide the movable contact 22 with an appropriate contact pressure against the stationary contact 26.
Simultaneous with the drawing of the movable iron core assembly 4 toward the stationary contact 26, the P coil of the electromagnetic coil 12 is deenergized and instead the other H coil of the electromagnetic coil 12 acts to hold the entire electromagnetic coil 12 in an energized state.
Also, in accordance with the movement of the movable iron core assembly 4 toward the stationary iron core 9, one end (the upper end in FIGS. 3 and 4) of the shift lever 3, being engaged with the hook 5 connected with the movable iron core assembly 4 by means of the spring 18, is pulled to pivot around a fulcrum or pivot point at which the shift lever 3 abuts against the spacer 7. Therefore, a pinion gear 30, axially slidably fitted over the rotary shaft 31 of the starter motor M for rotation therewith, is forced by the other end (the lower end in FIG. 3) of the shift lever 3 to displace axially on the rotary shaft 31 of the starter motor M, whereby the pinion gear 30 is brought into engagement with a ring gear 32 operably connected with an engine (not shown) with the result that the engine is turned over to be started. Upon starting of the engine, the key switch (not shown) is opened to deenergize the electromagnetic coil 12 so that the magnetic attraction force acting between the stationary iron core 9 and the movable iron core 16 collapses to permit the movable iron core assembly 4 to return to the initial position as illustrated in FIG. 4 under the action of the return spring 25.
With the conventional electromagnetic switch as constructed in the above-described manner, however, the clearance c between the radially inner peripheral surface 21a of the shaft member 20 is opened up upon closing of the stationary and movable contacts 26 and 22 so that powder, produced by the friction of repeated opening and closing of these contacts 26 and 22, is liable to come through the clearance c into the sliding surfaces between the bushing 14 and the movable iron core 16, as a consequence of which the sliding motion of the movable iron core 16 relative to the bushing 14 is impaired and further outside water tends to penetrate through the clearance c into a space or chamber S defined by the stationary and movable iron cores 9 and 16, the bushing 14 and the shaft member 20.